1. Field of the Invention
The present invention relates generally to a structure for improving the performance of various optical devices used for light intensity modulation, light switching, plane of polarization control, propagation mode control, optical phase matching control for second harmonic generation and other optical controls, and manufacturing method of the same.
2. Description of the Prior Art
Optical waveguide devices, having been conventional developed, have optical waveguide paths formed on transparent dielectric substrates (such as lithium niobate (LiNbO.sub.3)). These optical waveguide paths are variously shaped and associated with appropriate electrodes to control or modulate lights passing through therein by using electrooptic effect. For example, R. Alferness discloses this type of optical waveguide modulator in the paper, "Waveguide Electrooptic Modulators", IEEE Transactions on Microwave and Techniques, Vol. MTT-30, No.8, 1121-1137(1982). Furthermore, I. Kaminow discloses various manufacturing methods for this kind of optical waveguide device in the paper, "Optical Waveguide Modulators" IEEE Transactions on Microwave and Techniques, Vol. MTT-23, No.1, 57-70(1975). These papers disclose various optical waveguide devices.
In one such manufacturing method, lithium niobate or lithium tantalate is heat-treated at a high temperature to modify the refractive index of the material by out-diffusing the lithium. Alternatively, a metallic film of, for example, titanium is formed by vacuum evaporation and thermally diffused at a high temperature to raise the refractive index of the diffused area slightly above that of the surrounding area. In either case, the difference in refractive indices is used to confine light. An example of a Mach-Zehnder type optical modulator using a titanium diffusion is described in Unexamined Japanese Patent Application No. 63-261219/1988.
In another method, a metallic mask is formed over the specified areas and a proton-ion exchange is induced in phosphoric acid at 200.degree. C. to 300.degree. C., partially modifying the refractive index and forming the optical waveguide. Manufacturing methods that rely on out-diffusion, thermal diffusion, or ion exchange from the surface all form the optical waveguide by means of diffusion from the surface. The cross section of the optical waveguide is therefore necessarily determined by the diffusion process, resulting in numerous problems.
One of the biggest problems is coupling loss between the optical waveguide and the optical fiber. While the cross section of an optical fiber is circular, the shape of the most conventional optical waveguide is roughly an inverted triangle due to the formation of the waveguide by diffusion from the surface. Because the strength of the guided light is greatest near the surface, optical coupling with the optical fiber is poor, resulting in significant loss. Reducing this optical coupling loss is therefore an extremely important topic in optical waveguide device design.
Another problem caused by diffusion processing is greater optical propagation loss after diffusion processing than before. With titanium diffusion optical waveguide, for example, propagating loss of more than 1 dB/cm normally occurs. Reducing propagating loss is therefore another major topic in optical waveguide device design.
A third problem is the increase in optical damage resulting from diffusion processing. Optical damage refers the increase in propagation loss over time when a high intensity light source or a short wavelength light source is input to a diffusion-type optical waveguide. This is believed to be caused by the diffusion of ions in the optical waveguide resulting in increased trapping of electrons in the optical waveguide.
It should be noted that methods for forming an optical waveguide without relying on diffusion processing have been described. One of these is described by Kaminow (see above reference). In this method, lithium niobate crystals are grown on top of a lithium tantalate layer, or a lithium niobate thin-film is formed by sputtering on top of a lithium niobate or lithium tantalate layer, and the optical waveguide is formed in this lithium niobate top layer. A similar method is described in Unexamined Japanese Patent Application No. 52-23355/1977. This method also forms an epitaxial growth lithium niobate top layer over a substrate of lithium tantalate (e.g.) using liquid phase, gas phase, fusion, or other method, and forms the optical waveguide in this top layer. There are, however, several problems with these optical waveguide formation methods using such thin-film crystal growth technologies. First, it is difficult to obtain a thick-film in epitaxial growth film, and productivity is accordingly poor, because of the growth speed and flaws occurring in the crystals while being grown. In addition, the coupling characteristics of a thin film less than 5 micron thick with an optical fiber having a core diameter of approximately 10 microns are also poor. (The fiber core being where the light is confined.)
Productivity is further hampered because a good quality single crystal thin-film cannot be obtained unless the lattice constants of the thin-film is essentially the same as those of the substrate. It is therefore extremely difficult to form a good lithium niobate thin-film on a lithium tantalate substrate, and a mixed niobium-tantalum crystal film is often used. Pure lithium niobate, however, offers superior overall optical waveguide characteristics when compared with a mixed crystal film.
To increase the thickness of the growth layer, it may be possible to use the same material between the growth layer and the substrate. But, the growth layer and substrate if made by the same material will have the same crystal orientation. Due to the same crystal orientation, no satisfactory difference will be obtained in the indices of refraction between the growth layer and substrate.